This invention relates to an asynchronous multiplexer-demultiplexer for transmitting pulse code modulated (PCM) data over a communication link, and more particularly to a system for telemetering over a communication link using asynchronous multiplexers and demultiplexers.
Most telemetric systems now use digital techniques for data transmission. Since sampling and converting the sample to an n-bit word may be completely asynchronous, a system capable of accepting a plurality of completely asynchronous sources is required. It is recognized that once the data from a source has been converted to digital form, it is synchronous with a data clock provided for that source. The problem then is to encode the data clock of each source at the input to the transmitter and reconstruct the data clock at the output of the receiver. In the case of multiple sources, the system must be able to transmit multiple asynchronous inputs, and their respective clocks, and somehow multiplex them to provide a single transmission over one communication link, such as a laser beam.
In previous telemetric systems, time-division multiplexing has been provided with fixed sequencing for a number of data sources operating at different data clock rates. The sequencing was artificially designed into the system by calculating the position, and the percentage of time, that each source must have in the transmitted stream of data. The problem with such a fixed sequencing scheme is that data from the sources must be coordinated so that each source will have its data ready for transmission when its slot occurs. Either the data sources are synchronous, or their data is converted to a synchronous form. The problem with such a technique is that the time slot for each block of data transmitted for each source must be calculated for every specific input frequency and therefore limits the system's flexibility in accepting new or difficult data inputs. The need is for a system with greater flexibility in accepting a large variation in data clock rates without redesign, i.e., a generalized multiplexing scheme that does not have to be designed to a specific configuration of data input rates.